Positioning is a common requirement for many applications such as robot control, fire fighting, and entertainment. Many conventional methods for determining the position of an object are based on external reference points such as those described in U.S. Pat. Nos. 6,707,424 and 6,646,596.
The reference points may be active, such as a transmitter or receiver, or passive such as a reflector. The position of the device may be calculated by measuring the distance, delay and/or direction from the reference point to the device. Whilst these methods for determining the position of a device may perform well, the requirement of pre-setting the reference points is not generally convenient and may even be impossible in some situations.
Many conventional methods use a transmitter/receiver device in conjunction with at least one active/reflective device situated at a pre-determined position as a point of reference. Frequently, a system is provided which is capable of monitoring its own position relative to the active/reflective device, and this system is attached to the object. The best known of such systems is the GPS system, using satellites.
U.S. Pat. Nos. 5,977,958 and 6,054,950 describe methods for measuring time-of-arrival with ultrashort RF pulses (UWB) transmitted from a transmitter to an array of receivers. Despite the differences in technical details, both patents require devices at pre-known positions. U.S. Pat. No. 5,977,958 uses four receivers for its 2D scenario and U.S. Pat. No. 6,054,950 needs at least four receivers or beacons for 3D applications (these receivers or beacons are here termed “explicit references”). There are other similar technologies which are based on angles of arrival of received signals instead of their times of arrival. In such technologies, multiple references are needed. Again, the problem with this type of system is the requirement for references at pre-known positions, which increases the number of devices needed and introduces difficulty in setting up the systems in certain situations or surroundings. Furthermore, such systems require direct line of sight (LOS) between the transmitter and receivers, and the performance decreases sharply in the areas where LOS is not available.
Self-positioning methods employed in robotics often use a ring of ultrasonic transducers or laser range finders to obtain a measurement of the respective distance from an object to a surrounding environment in each of a number of directions. A sonar/laser range image may be formed from the distances between the detector and nearby objects. This image may be compared to a known floor plan or trained database in order to find the location of the robot. However, the known floor plan information and trained database may not be available in some environments.
Another U.S. Pat. No. 6,112,095 proposes a method of location determination in which a transmitter transmits a radio signal and a receiver device uses an array of antennas to receive the signal both directly and along paths which include reflections (multipath signals). Different locations of the transmitter cause the set of reflected paths to differ, so the received signals constitute a signature of the position. The determination of the location of the transmitter is based on a set of pre-calibrated signal covariance matrices corresponding to possible transmitter locations. However, the uniqueness of the signature is, in principle, not guaranteed and sometimes leads to large errors when the multipath features of one location are similar to those of other locations. Furthermore, the teaching in this citation is intended for outdoor application, and a base-station is required for its implementation.
A type of self-positioning device for vehicle use is described in U.S. Pat. No. 6,671,622. Another form of positioning system is able to determine its location without the need of external references, by using a north finder, an inertial measuring system, a velocimeter and an odometer. Such systems are very convenient, but the calculated position may not be sufficiently accurate.
In view of the foregoing problems with conventional methods and devices, a need exists for an easily applied self-positioning method which does not require the pre-setting of external reference points.